(1) Field of the Invention
The present invention relates to a device and to a method for regulating a turbine engine, in particular of a rotorcraft, making it possible to control the acceleration of the engine.
(2) Description of Related Art
This type of engine is fitted with a regulator system having the main function of regulating, in flight, the power that is delivered by the engine. The speed of rotation of the main lift or indeed propulsion rotor of the rotorcraft is thus maintained at a substantially constant value.
A rotorcraft is piloted in particular by acting on the pitch of the blades of the main rotor. An increase in the pitch causes a rapid drop in the speed of rotation of the rotor due to the increase in the drag of the rotor. The engine must then be accelerated rapidly in such a manner as to compensate for this drop so that the rotorcraft can be kept stabilized in flight. Likewise, when the pitch of the blades is decreased, it is necessary to decelerate the engine so that the speed of rotation of the rotor does not exceed the limit set by the manufacturer.
Furthermore, the engine must be regulated so as to enable the main rotor to absorb more power, but without running the risk of the engine surging. “Surging” is a phenomenon that affects the compressors of turbine engines when, locally, an excessive angle of incidence of a blade or a vane gives rise to aerodynamic separation, thereby considerably reducing the flow rate of air. One consequence of this phenomenon is overheating in the combustion chamber, which can lead to damage to a turbine of the engine.
Furthermore, engine surging may lead to a loss of power, to the appearance of vibration likely to damage bearings and to reduce passenger comfort for example, to the appearance of abnormal noises, or to the emission of flames.
Surging may also lead to engine flameout.
Likewise, decelerating must also be controlled so as to avoid engine flameout.
A Full Authority Digital Engine Control (FADEC) electronic regulator device is known. Regulation relationships, e.g. for regulating acceleration or deceleration, are programmed into the electronic regulator device so as to regulate the engine, but without running the risks of the engine surging or of flameout.
Furthermore, the electronic regulator device receives signals from sensors that measure various parameters of the engine. As a function of such parameters, the electronic regulator device uses regulation relationships so as to control an actuator that adjusts the fuel flow rate by acting on the position of the fuel flow rate metering unit.
Conventionally, engines are designed to operate without surging in their utilization envelope.
Under such circumstances, a regulation relationship for normal operation is established so as to present a first margin that is large relative to a surge zone of the engine.
Furthermore, a regulation relationship for acceleration is established. The acceleration regulation relationship presents a second margin that is small relative to a surge zone of the engine.
The acceleration regulation relationship may take into consideration air being extracted from the gas generator of the engine at a constant and maximum rate, or indeed mechanical power being extracted (known as power take-off (PTO)) from the gas generator of the engine at a constant rate.
In a graph plotting the compression ratio of the gas generator of an engine up the ordinate axis against the flow rate of air from the gas generator along the abscissa axis for a transient acceleration stage, an operating point of the engine moves away from a normal operating line that corresponds to the normal regulation relationship and reaches an acceleration operating line that corresponds to the acceleration regulation relationship. Reference may be made to the literature to obtain such a diagram.
The acceleration operating line is thus arranged between the normal operating line and a surging line representing the surging state of the engine.
The surge margin of the engine thus decreases when the engine accelerates. As a function of the intensity of the acceleration, a surge phenomenon is sometimes seen to appear.
In order to limit such phenomena, a system provided with a discharge valve may be implemented.
Likewise, a system known under the acronym IGV for “Inlet Guide Vanes” may be implemented.
Such systems make it possible to increase surge margins.
The prior art thus includes a single acceleration regulation relationship, possibly taking into consideration constant air extraction or constant mechanical power extraction (or “take-off”) from the gas generator of the engine, and a plurality of systems that may be activated so as to avoid a surge phenomenon appearing.
Document EP 1 712 761 describes an engine provided with electrical equipment. The electrical equipment makes it possible to extract power so as to improve a surge margin.
The technological background also includes documents U.S. Pat. No. 6,364,602, US 2006/042252, US 2006/101826, GB 2 251 657, and FR 2 968 716.